With the increasing threat of global energy shortage and serious environmental problems, renewable energy has been developing fast. Photovoltaic industry has an extensive future as its resources are plentiful and widespread. It is an important issue to lower cost and improve efficiency for photovoltaic system.
Inverter is used to convert DC from photovoltaic arrays into AC in photovoltaic system. There are two kinds of grid-connected inverters. They are isolated and non-isolated ones. The transformer is used in the former to keep human electrically safe. But because of its power loss and huge volume, it results in many problems, such as low efficiency and power density, and high cost. Therefore, the non-isolated inverter in photovoltaic system is popular. However, there is a common mode loop circuit for the absence of transformer in the non-isolated inverter system. The loop circuit produces leakage current which is dangerous to devices and human. So the leakage current becomes a critical issue to be solved for the non-isolated inverter system. At the same time, high efficiency is becoming another goal for the photovoltaic industry.
It is required to meet a wide range of input voltage for the non-isolated inverter as there is a huge scope of the fluctuation of the output voltage from photovoltaic arrays. However, there is a minimum input voltage for the non-isolated inverter working normally in photovoltaic system. It is possible for the output voltage from photovoltaic arrays to be less than the minimum input voltage. So a boost circuit is usually used in the photovoltaic inverter system.
As is known, there is a two-stage circuit in the present photovoltaic inverter system shown in FIG. 1. The first stage is DC-DC unit which usually employs the boost circuit. The second stage is DC-AC unit. When the output voltage from photovoltaic arrays is less than the minimum input voltage, the DC-DC unit works to boost the input voltage so that the DC-AC unit can work normally; otherwise, the DC-DC unit is bypassed while the DC-AC unit works normally. The two stages are connected in series so that it lowers efficiency of the whole system and increases cost.
A single stage full bridge inverter shown in FIG. 2 is disclosed in the reference of “Derivation, Analysis, and Implementation of a Boost-Buck Converter-Based High-Efficiency PV Inverter”. However, it costs much in case of multiple photovoltaic arrays. In addition, the efficiency is low when there are great differences among the numbers of photovoltaic cells in multiple photovoltaic arrays. The output voltage from each photovoltaic array is required to be same because they are connected in parallel. So the output voltage of each photovoltaic array is raised in various degrees. It lowers the system efficiency. As is well known, it is difficult for the full bridge inverter to cancel substantially the high frequency leakage current because of the parasitic factors within the inverter system. Furthermore, it costs much because it needs two AC filtering inductors and their magnetic cores are not common for the full bridge structure.